1, symbol set: empty set is recorded as _; The subset is marked as s _ t；; The intersection point is marked as A∩B (or b ∩ a); And record it as A∪B (or b ∪ a); The relative complement set is marked as A-B or a \ b；; The absolute complement set is marked as a' or _u(A) or ~ a.2. Set is a basic concept in mathematics and the main research object of set theory. The basic theory of set theory was founded in19th century.